In the reconstruction of damaged ligament in orthopedics, a method where autologous tendon is passed through a bone tunnel opened in bone and fixed thereto has been widely used.
In such a method, adhesion of tendon which is a soft tissue to bone which is a hard tissue is essentially bad and, therefore, adhesion and fixation of the transplanted tendon to the bone tunnel are bad and that is a cause which greatly affects the initiation of rehabilitation after the operation and retards its recovery.
For improvement of adhesion of biotissue to bone as such, there has been known a method in which biotissue, medical material, etc. are soaked in a calcium solution and a phosphate solution alternately so that a calcium phosphate compound is coated on the surface of the material to enhance the adhesion of biomaterial to bone (Japanese Patent Laid-Open 2000/327,314).
Further, the present inventors developed a method for fixing a calcium phosphate compound on the surface of tissue substrate for tendon or ligament which is biotissue in accordance with the above method and filed an application (Japanese Patent Application 11/323,753).
Such a method is a method which comprises a step where tissue substrate for tendon or ligament is alternately soaked in a calcium solution containing calcium ion but containing substantially no phosphate ion and a phosphate solution containing phosphate ion but containing substantially no calcium ion so that a calcium phosphate compound is formed and fixed at least on the surface of the substrate.
According to such a method however, when the biotissue substrate is alternately soaked in the above-mentioned solutions, pH of the solutions lowers as a result of formation of a calcium phosphate compound and there is a problem that, when a calcium phosphate compound is directly coated on the biotissue substrate, damage of the biotissue substrate is resulted.
Further, with regard to the osmotic pressure of such solutions, a solution having an osmotic pressure which is near that of body fluids is to be used and, when only a compound having a buffer effect which is commonly used is used, there is a limitation for its use because of the relation between the buffer effect and the osmotic pressure. Thus, it is the current status that a method for coating a calcium phosphate compound satisfying all of the requirements and having high safety to living body has not been found yet.
In view of the current status as mentioned above, the present inventors have repeatedly carried out intensive investigations for a method of coating the biotissue substrate where a calcium phosphate compound is coated on the biotissue substrate in which it is possible to form a uniform coat mainly comprising hydroxyapatite causing no damage of tissue to the biotissue substrate and, moreover, a soaking solutions satisfying the condition in relation to the osmotic pressure of body fluids are used.